The present invention generally relates to micromachined devices, and particularly microelectromechanical system (MEMS) devices formed by etching processes. More particularly, this invention relates to a micromachining process and design elements for a MEMS device using a deep reactive ion etching (DRIE) process to improve yields and device reliability.
A wide variety of MEMS devices are known, including accelerometers, rate sensors, actuators, motors, microfluidic mixing devices, springs for optical-moving mirrors, etc. As an example, rotational accelerometers that employ MEMS devices are widely used in computer disk drive read/write heads to compensate for the effects of vibration and shock. Other applications for rotational accelerometers that use MEMS devices include VCR cameras and aerospace and automotive safety control systems and navigational systems. Rotational rate sensors and accelerometers have been developed whose MEMS devices are fabricated in a semiconductor chip. Notable MEMS devices that employ a proof mass for sensing rotational rate or acceleration include a plated metal sensing ring disclosed in U.S. Pat. No. 5,450,751 to Putty et al., and an electrically-conductive, micromachined silicon sensing ring disclosed in U.S. Pat. No. 5,547,093 to Sparks, both of which are assigned to the assignee of this invention. Sparks"" sensing ring is formed by etching a chip formed of a single-crystal silicon wafer or a polysilicon film on a silicon or glass handle wafer. A sensor disclosed in U.S. Pat. No. 5,872,313 to Zarabadi et al., also assigned to the assignee of the present invention, employs a sensing ring and electrodes with interdigitized members. The positions of the interdigitized members relative to each other enable at least partial cancellation of the effect of differential thermal expansion of the ring and electrodes, reducing the sensitivity to temperature variations in the operating environment of the sensor. Each of the above sensors operates on the basis of capacitively sensing movement of their rings. The sensing rings are supported by a central hub or pedestal. Surrounding the rings are drive electrodes that drive the rings into resonance, while sensing electrodes that also surround the rings serve to capacitively sense the proximity of the ring (or nodes on the ring) which varies due to Coriolis forces that occur when the resonating ring is subjected to rotary motion.
Another notable MEMS device that employs a silicon proof mass for sensing rotational acceleration is disclosed in U.S. patent application Ser. No. 10/410,712 now U.S. Pat. No. 6,257,062 to Rich, incorporated herein by reference. Rich discloses a disk-shaped proof mass supported above a cavity formed in a substrate. Instead of being centrally supported by a pedestal, Rich""s proof mass is suspended from its perimeter with tethers anchored to the substrate rim surrounding the proof mass. The tethers allow the proof mass to rotate about an axis perpendicular to the plane containing the proof mass and tethers. Fingers extend radially outward from the proof mass and are interdigitized with fingers extending radially inward from the substrate rim. The cantilevered fingers of the proof mass and rim are capacitively coupled to produce an output signal that varies as a function of the distances between adjacent paired fingers, which in turn varies with the angular position of the proof mass as it rotates about its axis of rotation in response to a rotational acceleration.
Sensors of the type described above are capable of extremely precise measurements, and are therefore desirable for use in a wide variety of applications. However, the intricate proof masses and associated sensing structures required for such sensors must be precisely formed in order to ensure the proper operation of the sensor. For example, Rich""s device requires a sufficient gap between paired interdigitized fingers to prevent stiction and shorting, yet paired fingers must also be sufficiently close to maximize the capacitive output signal of the sensor. Rich employs stiction bumps formed on the proof mass fingers to inhibit stiction between closely-spaced fingers. Increasing the area of the fingers to achieve greater capacitive coupling would result in increased capacitive output for a given finger gap. However, traditional etching techniques have not generally been well suited for mass-producing silicon micromachines with high aspect ratios necessary to etch closely-spaced fingers in a relatively thick substrate. For example, with conventional etching techniques it is difficult to achieve a 10:1 aspect ratio capable of forming interdigitized fingers spaced three micrometers apart in a silicon substrate that is thirty micrometers thick. In addition to operational considerations, there is a continuing emphasis for motion sensors that are lower in cost, which is strongly impacted by process yield, yet exhibit high reliability and performance capability. Consequently, improvements in the processing of MEMS devices for sensing and other applications are highly desirable. Deep reactive ion etching (DRIE) is a process known as being capable of performing deep, high aspect ratio anisotropic etches of silicon and polysilicon, and is therefore desirable for producing all-silicon MEMS of the type taught by Rich. However, DRIE is a young technology practiced largely for research and development. Accordingly, to take advantage of the unique capabilities of the DRIE process, its etch idiosyncrasies must be determined and reconciled to render it suitable for mass production.
The present invention provides a process and design elements for a microelectromechanical system (MEMS) device by a deep reactive ion etching (DRIE) process during which a substrate overlying a cavity is etched to form trenches that breach the cavity to delineate suspended structures. The invention is particularly useful in the fabrication with a DRIE process of semiconductor MEMS devices used to sense motion or acceleration, which typically include a proof mass suspended above a cavity so as to have an axis of rotation perpendicular to the plane of the proof mass, as taught by Rich, Sparks and Zarabadi et al. While the invention will be illustrated in reference to a MEMS device with a proof mass, the invention is applicable to essentially any suspended structure that can be fabricated by forming a trench in a substrate overlying a cavity.
According to the invention, in addition to a relatively large member such as a proof mass, MEMS devices also may include additional and smaller structures that are suspended above the same cavity, such as the tethers and cantilevered fingers of Rich. A first general feature of the invention is the ability to define suspended structures with a DRIE process, such that the dimensions desired for the suspended structures are obtained. A second general feature of the invention is the ability to define specialized features, such as stiction bumps that, if delineated by DRIE, must be properly located between suspended structures in order to be effective in improving the reliability of the MEMS device. Yet another general feature of the invention is the control of the environment surrounding suspended structures delineated by DRIE in order to obtained their desired dimensions.
A significant problem identified and solved by the present invention is the propensity for the DRIE process to etch suspended features at different rates. DRIE has been determined to etch wide trenches more rapidly than narrower trenches. According to the invention, DRIE etches or, more accurately, erodes suspended structures more rapidly at greater distances from anchor sites of the substrate being etched, which occurs when a suspended structure becomes isolated from the bulk substrate when the trench(s) that delineates the structure breaches the cavity. (As used herein, an anchor site is a location on the bulk of the substrate from which the suspended structure is ultimately supported from the bulk of the substrate.) Consequently, though two suspended structures are separated by a gap of constant width, DRIE processes have been determined to more rapidly erode the suspended structure located farther from an anchor site. Using Rich""s MEMS device as an example, once the proof mass is separated from the bulk of the substrate using a DRIE process. the proof mass fingers etch more rapidly than the rim fingers because the rim fingers are anchored (cantilevered) directly from the rim of the bulk substrate surrounding the proof mass, while the proof mass fingers are ultimately anchored to the rim of the bulk substrate through the tethers that suspend the proof mass from the same rim of the bulk substrate. A consequence of this more rapid etch is backside erosion and lateral thinning of the proof mass fingers.
In view of the above, in order to DRIE etch a substrate above a cavity to form suspended structures above the cavity, in which a first of the suspended structures is farther from the substrate anchor site than the second suspended structure, the present invention exploits the greater propensity for backside and lateral erosion of certain structures farther from substrate anchor sites so that, at the completion of the etch process, all suspended structures have acquired their respective desired widths. In this example, first and second surface regions of the substrate corresponding to the first and second suspended structures are masked in preparation for the DRIE etching process, leaving exposed those surface regions of the substrate corresponding to the trenches that will surround and delineate the suspended structures. The first masked surface region is intentionally wider than the desired width of the first suspended structure, thus resulting in the adjacent exposed surface region being narrower than the width desired for the trench that will delineate the first suspended structure. The first and second suspended structures are then concurrently DRIE etched. According to the invention, as a result of the first suspended structure being a greater distance from the anchor site than the second suspended structure, the first suspended structure is subject to backside and lateral erosion after the cavity is breached, causing the first structure to narrow and eventually acquire its desired lateral width during completion of the etch. As a result, the first masked surface region is intentionally undercut so that, at the completion of the etch process, the first and second suspended structures have acquired their respective desired widths.
The tendency for DRIE to etch wider trenches more rapidly than narrower trenches, a phenomenon which may be termed xe2x80x9cetch lag,xe2x80x9d is also detrimental to the formation of suspended features for the same reasons explained above. An example is where first and second suspended structures are to be DRIE etched in a substrate over a cavity, in which the first structure is delineated by a wider trench than the second structure. Because the wider trenches of the first structure etch more rapidly during DRIE etching, the wider trenches breach the underlying cavity before the narrower trenches of the second structure, leading to backside erosion and lateral narrowing as explained previously. This phenomenon would be compensated for by masking the substrate so that the masked surface region corresponding to the first structure is intentionally wider than desired, resulting in the adjacent exposed surface regions being narrower than the width desired for the trench that will delineate the first structure, yet wider than the exposed surface regions of the substrate corresponding to the trench that will surround and delineate the second structure. The trench formed in the wider exposed surface regions of the first structure breaches the underlying cavity before the narrower trench of the second structure, with the resulting backside and lateral erosion of the first structure causing the first structure to narrow and eventually substantially acquire its desired width during completion of the etch of the narrower trench surrounding the second structure. However, this scenario is complicated by the findings of the present invention that backside and lateral erosion occur more rapidly with those suspended structures located farther from an anchor site. This invention provides two approaches for addressing this problem. A first is to taper the width of the mask for a suspended structure while maintaining a constant gap width for the exposed surface area in which the trench will be etched to delineate the structure. The mask is tapered to be wider with increasing distance from the anchor site, so that as the width of the mask increases, backside and lateral erosion is correspondingly more rapid to eventually produce a substantially uniform width for the structure. Alternatively, the width of the mask for a suspended structure is maintained constant while tapering the gap width for the exposed surface area in which the trench will be etched to delineate the structure. The gap is tapered to be wider with decreasing distance from the anchor site, so that backside and lateral erosion of the structure that occurs more rapidly with increasing distance from the anchor site is balanced by the more rapid etch rate associated with the increasingly wider gap near the anchor site. As a result, a substantially uniform width for the structure can again be obtained.
The teachings of this invention concerning the relationship between distance to an anchor site and backside and lateral erosion is also pertinent to other aspects of DRIE etching a MEMS device. As previously noted, a feature of the invention is the ability to properly define specialized elements, such as stiction bumps. According to the invention, stiction bumps must be defined in regions of the substrate away from those areas in which accelerated backside and lateral erosion will occur. Also a feature of this invention is maintaining a proper environment surrounding suspended structures, such as by eliminating unnecessary variations in trench width. An important example is avoiding intersecting trenches that would create a localized wider gap prone to more rapid etching and subsequent backside and lateral erosion, resulting in vertical notches at the intersections.
In view of the above, it can be seen that the present invention provides a DRIE etching process by which suspended structures of desired widths can be more precisely formed. As a result, the present invention is able to take advantage of the deep etching capability of the DRIE process, while compensating for etch idiosyncrasies that would otherwise adversely affect the structural integrity and durability of a MEMS device, so as to improve yields and device reliability.
Other objects and advantages of this invention will be better appreciated from the following detailed description.